Electrical resistance device and method of making it



Aug. 28, 1956 B. B. SCOTT 2,

ELECTRICAL RESISTANCE DEVICE AND METHOD OF MAKING IT Filed July 15, 1955TEFEI IN V EN TOR. BE/Vd/MV/A/ 3. 5 C 0 T United States PatentELECTRICAL RESISTANCE DEVICE AND METHOD OF MAKING IT Benjamin B. Scott,Schenectady, N. Y., assignor to the United States of America asrepresented by the Secretary of the Air Force Application July 15, 1955,Serial No. 522,405

6 Claims. (Cl. 201-48) This invention relates to a resistance cardhaving straight or curved elevated brush-contacting areas. These newresistance cards are designed for side-brushing and are useful inprecision potentiometers incorporated in electrical analogue computers.

The new side-brush resistance-card potentiometer represents a majoradvantage in quality over conventional potentiometers of similarconfiguration and permits the manufacture of a product having qualitycomparable with that of the standard edge-brushed type. It provides thedesigner with considerable additional latitude in the type ofpotentiometer mounting and operating arrangement he may Wish to selectfor use. Also it has permitted the solution of hereto unsolved brushingproblems on siderushed types of precision otentiometers.

It was found in earlier practice that the firm fastening ofresistance-card wire-turns to cards for preventing movement or flexureunder the influence of brush contact in passing produced thousand-foldincrease in brushinglife operation expectancy. Institution and use of athermo-setting resin-bonding process for firmly'attaching wire turns tocards on resistance card components of edge-brushed potentiometers wasresponsible for increasing average brushing life from 10,000 operationsto 10,000,000 operations and has, of course, been standardized in ourprocessing. However, in attempting to apply this bonding method toresistance cards intended for side-brushing it was found impossible tocontrol the height of the bond between wires to the limits of about 10%to 25% found satisfactory for edge brushed type of potentiometers.Instead wire-turns would be found immersed in the bond or be completelyunbonded or would be bonded to one another but not to the cardsupporting member.

The excess bond material between turns could not be removed by knowntechniques without substantially equal removal of metal from the wirewhich ruined the electrical quality of the resistance winding, withoutlowering the bond-height between turns sufficiently to provide brushcontacting without electrical noise. Where turns were loose, the contactpoints of brush members were found to penetrate between the turns to thecard surface and to cause winding breakage and card destruction inrelatively few brushing operations. Where turns were bonded to oneanother but not to the card winding support it generally occurred thatvoids between wire-turns existed at the ends of these sections or wereproduced by brushingsurface cleaning operations so that destructionunder relatively few brushing operations occurred similarly to the casefor unbonded turns.

Investigation of the poor bonding properties of the sides of woundresistance cards disclosed the lack of flatness in the cards used aswinding-forms produced nonuniform distribution of material and resultedin the unsatisfactory bonding conditions described above. Additionalinvestigation disclosed that a thin elevating strip of insulatingmaterial approximately 0.005 thick attached to the card winding formbefore the winding operation or placed in position between the card andwire-turns after the winding operation in the location of the desiredbrushing surface produced the uniform contact with wire-turns needed foruniform distribution of bonded material between the wire-turns and usingproper bond-curing techniques produced extremely uniform high-qualitybonding of wireturns to cards which will readily clean for brushcontacting in the area of the elevating strip. These finished flatresistance cards may be used flat or may be formed by conventionaltechniques into cylindrical shaped cards, which have elevating strip inthe brush-contacting area on the inner or outer surfaces or both. 7

It is an object of this invention to provide a method for making aresistance card having elevated straight or curved brush-contactingsurface on one or both sides of the card.

It is another object of this invention to provide a resistance cardhaving an elevated straight or curved brusheontacting surface on atleast one side of the card which card is usable in side-brushpotentiometers.

It is a further object of this invention to provide a flat resistancecard having elevated brush-contacting surface on one or both sidesthereof which card can be shaped by conventional methods into acylindrical card particularly useful for computer potentiometer.

These and other objects of the invention will become apparent as thedetail description of the invention proceeds.

The invention will be more clearly understood from the followingdetailed description of the following specific examples thereof read inconjunction with the accompanying drawings wherein:

Fig. 1 is a partial elevational view of an embodiment of the inventionat an uncompleted stage of development previous to the addition of thethermosetting resin but after the winding of the wire on the card;

Fig. 2 is a cross sectional view of the Fig. 1 embodiment taken alongthe lines 22 of Fig. 1;

Fig. 3 is a partial sectional view of the Fig. 1 embodiment of theinvention incorporated in the curing fixture;

Pig. 4 is a cross sectional view of the embodiment of Fig. 1 in acompletely finished condition;

Fig. 5 is a perspective view of the same embodiment of the invention asis shown in Fig. 1 but of the finished resistance card; and

Fig. 6 is an elevational view of another embodiment of my inventionwherein an elevated curved brush-contacting area is incorporated ratherthan an elevated straight brush-contacting area.

Fig. 1 shows an embodiment of my invention wherein an elevated straightbrush-contacting area is used. The resistance card of Fig. 1 has a basestrip 11 to which is attached an elevational strip 12. These dielectricstrips 11 and 12 are suitably made of varnish impregnated glass fabricmaterial used in making conventional resistance cards. The strips maysuitably be bonded together by a thermosetting resin. Suitable resinsfor bonding these strips are described in detail in U. S. 2,528,235. IAlternately elevated strip 12 could be formed as an integral and unitarypart of base strip 11. A coil of resistance wire 13 is wrapped aroundstrips 11 and 12 forming individual turns 14 thereon. Alternativelystrip l zjneed not have been previously bonded to strip 11 but. may heslipped under the wire turns after winding the wire on card 11 since thelater curing operations will bond thestrips and wire. The resistancewire used for these cards is resistance Wire conventionally used inmaking resistance cards. Normally this will be resistance wire which hasbeen coated with an insulating coating of a suitable alkyd, polyvinylformal resin, or a mixture thereof. vvFig. 1 shows the uncompletedresistance card, i. e., uncoinpleted to the extent that the themoplasticresin has not been added to bond the wire to the insulating strips andother finishing operations have not been performed on the card.

Fig. 2 is a cross sectional view taken along the lines 22 of Fig. 1.This view shows the relation of elevating strip 12 to base strip 11 andwire convolutions 14- prior to the addition of the thermoplastic resinto bond the Wire to the strips and prior to the curing operation.

The next step in the making of the resistance card is to add theflexible thermoplastic resin to the card suitably by dipping the card ina solvent solution of tie resin. The viscosity of the resin solution isadjusted by adjusting the concentration so excess resin solution runsoff the card. This is a conventional procedure. The card is then placedwith the elevation strip up and the excess resin is allowed to drainfrom the card. The card is allowed to air dry for a suificient time atambient temperatures to cause the resin to take an initial set by theevaporation of some of the solvent. This air-drying step is a desirablestep but not absolutely necessary and the next drying step at elevatedtemperature may be proceeded with directly after the dipping step.

The next step in forming the resistance card is to place the card havingthe air dried resin thereon in an oven at about 75 C. to remove themajor portion of the solvent. Obviously this temperature might besomewhat lower or higher depending on the particular solvent used forthe resin. The partially completed resistance card is now ready for thepressin operation.

Fig. 4 shows the oven dried resistance card placed in a curing fixture15. This curing fixture is assembled by placing a cellophane or othersuitable parting sheet 3.6 on a base pressing sheet 17 suitably made ofmetal as a support for the card. Next the resistance card is placed onthe cellophane parting sheet 16, a second cellophane parting sheet 17 isplaced over the resistance card, a silicone-rubber pressure-equalizingsheet 18 is placed on sheet 17, a third cellophane parting sheet 19 isplaced on sheet 18 and finally a metal cover plate 20 is placed on sheet19 to complete the assembling of curing fixture 15. This curing fixtureis now ready for the pressing operation. It is placed in a hot pressconventionally used for making the resistance cards of the prior art.The curing fixture containing the partially completed resistance card ispressed at about 50 pounds per square inch and at about 135 C. for aboutten minutes to cause the wire convolutions 14 to be firmly bonded to thedielectric strips as the resin takes on a permanent set. Theseparticular conditions as well as the drying conditions specified aboveare based on Permafil-Formvar thermosetting-resin bonding solution. Thisparticular resin solution is not at all essential to the invention andmany other resin solutions of similar nature would be satisfactory.Conditions of treatment would vary slightly depending on what resinsolution was used. Suitable resins are described in detail in U. S.2,528,235. In this pressing operation with the help of pressureequalizing sheet 18 convolntions 14 are pressed down as shown in Fig. 4,i. e., the wire is made to more closely conform to the shape of strips11 and 12. That is, the wire turns are forced down into hollows 11a and11]) causing this portion of the wire to be completely imbcdded in thebonding resin and so the wire is more rigidly attached to the strips.This is clearly shown in Fig. 4 which is a cross sectional view similarto Fig. 2 of the resistance card, except that Pig. 4 shows the cardafter hot-pressing and Fig. 2 before hot-pressing.

The last step in forming the resistance card consists of subjecting thewire turns directly over the brush-contacting area provided by strip 12to a conventional treatment with a felt-wheel and jewelers rougewire-cleaning technique for removal of the wire insulation and bondingmaterial to give bare wires for electrical contact. Fig. 5 shows aperspective view of the completed resistance card.

Fig. 6 shows a finished resistance card which has a curved, circular inthis case, brushing area elevation strip 21 on a base dielectric strip22. In a conventional fashion coil 23 having wire turns 24 has beenwound on the strips and the card has been treated in a manner similar tothat used on the card having the straight brushing path to form afinished resistance card shown.

Although the invention has been described in terms of specifiedapparatus and method which is set forth in considerable detail, itshould be understood that this is by way of illustration only and thatthe invention is not necessarily limited thereto, since alternateembodiments and operating techniques will become apparent to thosesltilled in the art in View of the disclosure. For example, the fiatresistance cards described may be formed by conventional techniques intocylindrical resistance cards. If elevational brushing strips have beenplaced on both sides of the card, both internal and externalbrush-contacting is provided for the potentiometer in which the card isused. Normally brush-contacting would be provided for on either theinner or outer surface of the cylinder but not both surfaces.Accordingly, modifications are contemplated which may be made withoutdeparting from the spirit of the described invention or the scope of theappended claims.

What is claimed is:

1. A method of making a resistance card comprising step 1 of bonding abase strip of dielectric material to a second elevational strip ofdielectric material shaped to conform to the brush-contacting area ofthe potentiometer for which the card is designed, step 2 of winding acoil of resistance wire around the product of step 1, step 3 of applyinga flexible thermosetting resin to the product of step 2 and removing theexcess resin, step 4 of drying ing the product of step 3 at atemperature and time sufficient to remove the major portion of thesolvent from the resin but not sufiicient to completely set the resin,step 5 of assembling the product of step 4 in a curing fixture, step 6of heating and pressing the fixture containing the product of step 4 ina hot-press at a temperature and for a time sufiicient to cause theresin to set and at a pressure sufficient to cause the wire to conformclosely to the surface of said strips including the wire adjacent theedges of said second strip and to hold the elements of the product ofstep 4 in close association until the resin is set, step 7 ofdisassembling the curing fixture preferably while hot to preventsticking difiiculties, and step 8 of removing insulation and resin fromthe brush contact surface of the wire to give the finished resistancecard.

2. The method of claim 3 wherein a unitary strip of dielectric materialhaving a raised brush-contacting area thereon is formed rather thanusing separate base and do vational strips and bonding them together.

3. A method of making a resistance card comprising step 1 of bonding abase strip of dielectric material to a second elevational strip ofdielectric material shaped to conform to the brush-contacting area ofthe potentiometer for which the card is designed, step 2 of winding acoil of resistance wire around the product of step 1, step 3 of applyinga flexible thermosetting resin to the product of step 2 and removing theexcess resin, step 4 of airdrying at ambient temperatures the product ofstep 3 for a time sufficient to evaporate some of the solvent and givethe resin an initial set, step 5 of drying the product of step 4 at atemperature sufficient to remove the major portion of the solvent fromthe resin but not sufficient to completely set the resin, step 6 ofassembling the product of step 5 in a curing fixture, step 7 of heatingand pressi the fixture containing the product of step 5 in a hot-pressat a temperature and for a time sufficient to cause the resin to set andat a pressure sufficient to cause the wire to conform closely to thesurface of said strips including the wire adjacent the edges of saidsecond strip and to hold the elements of the product of step 5 in closeassociation until the resin is set, step 8 of disassembling the curingfixture preferably while hot to prevent sticking difiiculties, and step9 of removing insulation and resin from the brush contact surface of thewire to give the finished resistance card.

4. A method of making a resistance card comprising step 1 of bonding abase strip of dielectric material to a second elevational strip ofdielectric material shaped to conform to the brush-contacting area ofthe potentiometer for which the card is designed; step 2 of winding acoil of resistance wire around the product of step 1; step 3 of applyinga flexible thermosetting resin to the product of step 2 and removing theexcess resin; step 4 of drying the product of step 3 at a temperatureand time sufiicient to remove the major portion of the solvent from theresin but not sufficient to completely set the resin; step 5 ofassembling the product of step 4 in a curing fixture by placing acellophane parting sheet upon a base pressing plate, placing the productof step 4 on the cellophane parting sheet, placing a second cellophaneparting sheet on top of the product of step 4 so it is sandwiched between cellophane parting sheets, placing a silicone-rubberpressure-equalizing sheet on the second cellophane parting sheet,placing a third cellophane parting sheet on the silicone-rubberpressure-equalizing sheet, and placing a metal cover pressing-plate onthe third cellophane parting sheet to complete the assembling of thecuring fixture; step 6 of heating and pressing the fixture containingthe product of step 4 in a hot-press at a temperature and for a timesufficient to cause the resin to set and at a pressure sufiicient tocause the wire to conform closely to the sur face of said stripsincluding the wire adjacent the edges of said second strip and to holdthe elements of the product of step 4 in close association until theresin is set; step 7 of disassembling the curing fixture preferablywhile hot to prevent sticking difficulties; and step 8 of removinginsulation and resin from the brush contact surface of the wire to givethe finished resistance card.

5. A method of making a resistance card comprising step 1 of bonding abase strip of dielectric material to a second elevational strip ofdielectric material shaped to conform to the brush-contacting area ofthe potentiometer for which the card is designed; step 2 of winding acoil of resistance wire around the product of step 1; step 3 of applyinga flexible thermosetting resin to the product of step 2 and removing theexcess resin; step 4 of airdrying at ambient temperatures the product ofstep 3 for a time sufficient to evaporate some of the solvent and givethe resin an initial set; step 5 of drying the product of step 4 at atemperature and time sufficient to remove the major portion of thesolvent from the resin but not sufficient to completely set the resin;step 6 of assembling the product of step 4 in a curing fixture byplacing a cellophane parting sheet upon a base pressing plate, placingthe product of step 5 on the cellophane parting sheet, placing a secondcellophane parting sheet on top of the product of step 5 so it issandwiched between cellophane parting sheets, placing a silicone-rubberpressure-equalizing sheet on the second cellophane parting sheet,placing a third cellophane parting sheet on the silicone-rubberpressure-equalizing sheet, and placing a metal cover pressing-plate onthe third cellophane parting sheet to compiete the assembling of thecuring fixture; step 7 of heating and pressing the fixture containingthe product of step 5 in a hot-press at a temperature and for a timesutficient to cause the resin to set and at a pressure sufficient tocause the wire to conform closely to the surface of said stripsincluding the wire adjacent the edges of said second strip and to holdthe elements of the product of step 5 in close association until theresin is set, step 8 of disassembling the curing fixture preferablywhile hot to prevent sticking difficulties, and step 9 of removinginsulation and resin from the brush contact surface of the Wire to givethe finished resistance card.

6. A resistance card comprising a base strip of dielectric materialhaving an elevational strip of dielectric material formed as a unitarypart thereof, said elevational strip shaped to conform to the brushingpath of the potentiometer for which the card is designed, a coil ofresistance Wire wound around said strips, the turns of said coil beingbonded to said strips by a flexible thermosetting resin and being shapedto conform closely to the surface of said strips including the wireadjacent the edge of said elevational strip, and the brush-contactsurface of the Wire is bare for electrical contact.

References Cited in the file of this patent UNITED STATES PATENTS2,557,790 Launey June 19, 1951

1. A METHOD OF MAKING A RESISTANCE CARD COMPRISING STEP 1 OF BONDING ABASE STRIP OF DIELECTRIC MATERIAL TO A SECOND ELEVATIONAL STRIP OFDIELECTRIC MATERIAL SHAPED TO CONFORM TO THE BRUSH-CONTACTING AREA OFTHE POTENTIOMETER FOR WHICH THE CARD IS DESIGNED, STPE 2 OF WINDING ACOIL OF RESISTANCE WIRE AROUND THE PRODUCT OF STEP 1, STEPT 3 OFAPPLYING A FLEXIBLE THERMOSETTING RESIN TO THE PRODUCT OF STEP 2 ANDREMOVING THE EXCESS RESIN, STEP 4 OF DRYING ING THE PRODUCT OF STEP 3 ATA TEMPERATURE AND TIME SUFFICIENT TO REMOVE THE MAJOR PORTION OF THESOLVENT FROM THE RESIN BUT NOT SUFFICIENT TO COMPLETELY SET THE RESIN,STEP 5 OF ASSEMBLING THE PRODUCT OF THE STEP 4 IN A CURING FIXTURE, STEP6 OF HEATING AND PRESSING THE FIXTURE CONTAINING THE PRODUCT OF STEP 4IN A HOT-PRESS AT A TEMPERATURE AND FOR A TIME SUFFICIENT TO CAUSE THERESIN TO SET AND AT A PRESSURE SUFFICIENT TO CAUSE THE WIRE TO CONFORMCLOSELY TO THE SURFACE OF SAID STRIPS INCLUDING THE WIRE ADJACENT THEEDGES OF SAID SECOND STRIP AND TO HOLD THE ELEMENTS OF THE PRODUCT OFSTEP 4 IN CLOSE ASSOCIATTION UNTIL THE RESIN IS SET, STEP 7 OFDISSEMBLING THE CURING FIXTURE PREFERABLY WHILE HOT TO PREVENT STICKINGDIFFICULTIES, AND STEP 8 OF REMOVING INSULATION AND RESIN FROM THE BRUSHCONTACT SURFACE OF THE WIRE TO GIVE THE FINISHED RESISTANCE CARD.